1. Field of the Invention
The present invention relates to a base station transmitter employing a CDMA (Code Division Multiple Access) scheme as a multiple access scheme, and more particularly to a power controlling method for transmission signals in a plurality of communication channels in the base station transmitter.
2. Description of the Prior Art
In a communication system with the CDMA, data to be transmitted is spectrum-spread modulated using a spread code (i.e., a pseudorandom noise (PN) sequence) in a transmitter and the spread modulated data is transmitted to a receiving side. In a receiver, the received data is demodulated by despreading the received data using the same spread code as the spread code used in the transmitter. In the CDMA communication system, since each communication channel is specified by selecting the spread code for use in despreading on the receiving side, a plurality of communication channels can be established in the same frequency band by a plurality of transmitting sides using different spread codes orthogonal to one another for spreading.
However, since it is difficult to completely maintain orthogonality between all of used spread codes, actually the respective spread codes are not completely orthogonal to one another but have components correlated with the other codes. These correlated components act as interference components for their own communication to contribute to deteriorated communication quality. Since such a factor causes interference components, the interference components are increased as the number of communication channels is increased. Thus, in the CDMA communication system, Eb/Io (ratio of desired received wave power to interference wave power) can be most effectively ensured when all of a plurality of signals to be received in a receiver has the same power. Therefore, the CDMA communication system must control transmission power on each transmitting side.
A base station in the CDMA communication system controls transmission power of each mobile station such that the reception power at the base station from each mobile station is equal to one another.
Additionally, a base station spread modulates transmission signals destined for each mobile station and then combines the spread modulated signals into a combined spread signal which is then transmitted to each mobile station. When a base station combines the plurality of signals, it controls the amplitudes of transmission signals destined for each mobile station in accordance with an amplitude controlling signal included in an upward channel from each mobile station, as opposed to the power control as described above.
To control the amplitudes of a plurality of signals and combine them in this manner, a conventional CDMA base station transmitter changes the amplitudes of spread modulated transmission signals with a variable gain amplifier or the like, and then combines the respective transmission signals. However, according to this method, since many variable gain amplifiers are required as communication channels, thereby having a disadvantage that an increased number of communication channels causes an increase in the scale of a circuit of a CDMA base station transmitter. Also, an analog circuit such as a variable gain amplifier requires considerable adjustment, maintenance, or the like, which involves time and effort. In view of the situation, a CDMA base station transmitter having transmission power controlling function such that an amplitude of each channel is controlled with digital processing without requiring a variable gain amplifier or the like has been employed.
As a CDMA base station transmitter having such a transmission power controlling function, a base station transmitter as described in Japanese Patent Laid-open Application No. 10-22977 (JP, 10022977, A) has been proposed, for example. FIG. 1 is a block diagram showing a configuration of a conventional CDMA base station transmitter having the transmission power controlling function.
The conventional CDMA base station transmitter shown in FIG. 1 spreads respective transmission signals for n communication channels #1 to #n with different spread codes, and then combines the respective signals into a combined spread signal which is then transmitted to receivers. The CDMA base station transmitter uses orthogonal modulation as a primary modulation before the spread modulation. Each transmission signal can be divided into an in-phase component I and a quadrature component Q. The CDMA base station transmitter, as shown in FIG. 1, has channel signal processing units 211 to 21n provided for respective communication channels, spread code generator 12 for generating spread codes for respective communication channels, spreading and combining unit 83 for performing spread modulation and combination, and amplitude controlling signal decoder 14.
Amplitude controlling signal decoder 14 is supplied with received signal 101 as an input which has been transmitted from each mobile station, and decodes and outputs amplitude controlling signals 1021 to 102n for respective communication channels #1 to #n included in received signal 101. Amplitude controlling signals 1021 to 102n indicate whether the transmission power of the CDMA base station transmitter is to be increased or to be reduced.
Channel signal processing unit 211, as shown in FIG. 2, comprises amplitude correction value generator 211, adder 212, memory 214, and parallel-to-serial converter 213. The configurations of channel signal processing units 212 to 21n are the same as that of channel signal processing unit 211. Amplitude correction value generator 211 receives amplitude controlling signal 1021 from amplitude controlling signal decoder 14 and calculates a correction value for correcting amplitude signal 104 in accordance with the direction of amplitude controlling signal 1021. More specifically, since amplitude controlling signal 1021 is provided for directing an increase or decrease in the amplitude of a transmission signal for communication channel #1, amplitude correction value generator 211 generates as a correction value a signal for increasing or decreasing the amplitude signal by a predetermined value in accordance with the direction of amplitude controlling signal 102. For example, when amplitude controlling signal 1021 directs an increase, a signal xe2x80x9c+1.0 dBxe2x80x9d is generated, while a signal xe2x80x9cxe2x88x921.0 dBxe2x80x9d is generated when amplitude controlling signal 1021 directs a decrease.
Adder 212 adds the correction value calculated at amplitude correction value generator 211 to amplitude signal 104 before correction stored in memory 214 to provide the result as amplitude signal 104 after correction. Memory 214 temporarily stores amplitude signal 104 provided from adder 212 and provides the signal to adder 212 after holding the signal for a predetermined time period. Parallel-to-serial converter 213 converts amplitude signal 104 provided from adder 212 and transmission signal 1031 into one serial signal and outputs the signal. The format of the serial signal is shown in FIG. 3.
The processing with channel signal processing unit 211 shown in FIG. 2 may be also realized with a DSP (Digital Signal Processor) using software.
Spreading and combining unit 83 comprises serial-to-parallel converters 321 to 32nfor receiving the serial signals from the channel signal processing units, spread modulation circuit 132 for performing spread modulation, and combining circuit 133 for generating a combined spread signal. In spreading and combining unit 83, each of the serial signals from respective channel signal processing units 211to 21n is applied to each of serial-to-parallel converters 321 to 32n, converted to a parallel signal, and separated into a transmission signal (I component and Q component) and an amplitude signal. The transmission signal separated from the serial signal with each of serial-to-parallel converters 321 to 32n is spread modulated at spread modulation circuit 132 with a spread code generated at spread code generator 12 to be outputted as a spread modulated signal. On the other hand, the amplitude signal separated from the serial signal with each of serial-to-parallel converters 321 to 32n is applied to combining circuit 133 for each of communication channels #1 to #n. Combining circuit 133 converts the spread modulated signal generated by spread modulation circuit 132 into a signal with an amplitude value indicated by the amplitude signal and then combines respective signals to generate a combined spread signal.
In this manner, the conventional CDMA base station transmitter shown in FIG. 1 is configured such that an amplitude signal indicating an amplitude value of a transmission signal is generated besides the transmission signal, and these signals are supplied as a serial signal to spreading and combining unit 83. In spreading and combining unit 83, signals each obtained by spread modulating the transmission signal with a spread code are combined using the amplitude values indicated by the amplitude signals.
As described above, amplitude controlling signals 1021 to 102n are transmitted from mobile stations through received signal 101 at regular intervals. An example of the format of received signal 101 is shown in FIG. 4.
FIG. 4 is a diagram showing the format of received signal 101 for communication channel #1. Transmission amplitude controlling signal 1021 is transmitted at intervals of 0.625 ms.
In the CDMA base station transmitter, since amplitude controlling signal decoder 14 requires a certain time period for decoding amplitude controlling signal 1021 from received signal 101, the CDMA base station transmitter requires a certain time period from the reception of received signal 101 to the update of amplitude signal 104 at channel signal processing unit 211. As a result, if the CDMA base station transmitter provides a transmission signal to a mobile station before amplitude signal 104 is updated, the newest amplitude controlling signal 1021 is not reflected in the amplitude value of the transmission signal. For example, referring to FIG. 4, when the CDMA base station transmitter receives amplitude controlling signal 1021 at time to, the CDMA base station transmitter must send a transmission signal at an amplitude value obtained by reflecting received amplitude controlling signal 1021 before time t4 at which a mobile station next transmits amplitude controlling signal 1021. Assuming that amplitude signal 104 is updated at time t2 since decoding requires a certain time period, amplitude signal 104 is a value before the update when it is transmitted from channel signal processing unit 211 to spreading and combining unit 83 at time t1. For this reason, amplitude signal 104 must be transmitted to spreading and combining unit 83 between time t2 at which it is updated with a new value and t4.
However, the serial signal transmitted from channel signal processing unit 211 to spreading and combining unit 83 includes the transmission signal (I,Q) and the amplitude signal, as shown in FIG. 3. This causes a disadvantage that a delayed timing of the transmission of the serial signal by channel signal processing unit 211 leads to a delayed timing of the transmission of the transmission signal (I,Q) to spreading and combining unit 83 as well. Since channel signal processing unit 211 can not transmit the serial signal at a substantially delayed timing to avoid the disadvantage, the aforementioned conventional CDMA base station transmitter can not quickly reflect the amplitude controlling signal from a mobile station in the control of the transmission power of the base station. Therefore, the accuracy of control of transmission power is low in the CDMA base station transmitter.
Additionally, in the conventional CDMA base station transmitter, since channel signal processing units 211 to 21n are connected to spreading and combining unit 83 with a one-to-one interface, there exists a disadvantage that an increased number of multiplexed communication channels accordingly increases the number of transmission lines between spreading and combining unit 83 and respective channel signal processing units 211 to 21n. In particular, for a CDMA base station transmitter which employs information such as voice at a low bit rate but requiring multiple channels as a transmission signal, an increased number of transmission lines causes an increased scale of a circuit.
It is an object of the present invention to provide a CDMA base station transmitter which can quickly reflect an amplitude controlling signal from a mobile station in transmission power control of a base station.
It is another object of the present invention to provide a CDMA base station transmitter in which a circuit scale is not increased, even with an increased number of multiplexed communication channels.
To achieve the aforementioned objects, in a CDMA base station transmitter according to the present invention, one or more dummy bits are inserted between a transmission signal and an amplitude signal. The use of the dummy bit in this manner makes it possible to delay a timing of transmission of the amplitude signal for each communication channel to a spreading and combining unit without delaying a timing of transmission of the transmission signal to the spreading and combining unit. As a result, an amplitude controlling signal transmitted from a mobile station can be reflected in the amplitude signal immediately after decoding, thereby enhancing accuracy of control of transmission power of a base station.
The other object of the present invention is achieved by providing channel block time division multiplexing means for performing time division multiplexing of first serial signals from a plurality of channel signal processing means provided for each communication channel into a second serial signal to output the second serial signal, and channel block separating means for separating the second serial signal from the channel block time division multiplexing means into the original first serial signals before the time division multiplexing. Specifically, transmission signals and amplitude signals for respective communication channels are multiplexed in a time division manner into one serial signal, i.e., the second serial signal, which is transmitted to combining means, thereby reducing the number of transmission lines to enable a reduction in the circuit scale.
For a signal format of the second serial signal, a signal format in which a transmission signal for each communication channel precedes and an amplitude signal for each communication channel is arranged following the transmission signal may be employed. The use of such a format makes it possible to delay a timing of transmission of the amplitude signal for each communication channel to a spreading and combining unit without delaying a timing of transmission of the transmission signal to the spreading and combining unit. As a result, an amplitude controlling signal transmitted from a mobile station can be reflected in the amplitude signal immediately after decoding, thereby enhancing accuracy of transmission power control of a base station.
Alternatively, for a signal format of the second serial signal, a signal format in which a transmission signal for each communication channel is first transmitted, one or more dummy bits are transmitted next, and finally an amplitude signal for each communication channel is transmitted may be employed. The use of the dummy bit in this manner makes it possible to delay a timing of transmission of the amplitude signal for each communication channel to a spreading and combining unit without delaying a timing of transmission of the transmission signal to the spreading and combining unit. As a result, an amplitude controlling signal transmitted from a mobile station can be reflected in the amplitude signal immediately after decoding, thereby enhancing accuracy of transmission power control of a base station.
Additionally, in the present invention, a plurality of channel signal time division multiplexers may be provided. Since such a configuration allows a larger number of communication channels to be spread and combined as compared with the case of a single channel signal time division multiplexer, the circuit scale can be substantially reduced by decreasing the number of transmission lines.